Image capturing system for correcting signals output from defective pixels

ABSTRACT

An image capturing system includes a signal correction unit which corrects a signal output from a defective pixel in an optical black region based on a signal output from a normal pixel. The optical black region has a plurality of pixel blocks. Each of the plurality of pixel blocks has a plurality of pixels each including one or more elements which have the same functions as in the remaining pixels and which have relative positions different from the remaining pixels. The signal correction unit corrects the signal output from the defective pixel in the optical black region based on a signal output from a normal pixel which is included in another pixel block different from the pixel block of the defective pixel in the optical black region and includes one or more elements having the same functions and same relative positions as in the defective pixel.

This application is a continuation of application Ser. No. 12/104,991filed Apr. 17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image capturing system, signalprocessing circuit, and signal processing method.

2. Description of the Related Art

Many image capturing systems such as a digital camera, video camera,copying machine, or a facsimile apparatus incorporate an image sensingapparatus in which pixels each including a photoelectric conversionelement are arrayed one- or two-dimensionally. The image sensingapparatuses include a CCD image sensing apparatus or an amplifying imagesensing apparatus having amplifying elements in the pixel region.

Recent image sensing apparatuses tend to increase the number of pixels.As the area of one pixel decreases, the area of a photoelectricconversion element also tends to decrease. That a photoelectricconversion element has a smaller area indicates that an amount ofincident light per pixel becomes small. In, for example, an amplifyingimage sensing apparatus having a plurality of elements in one pixel, thearea occupied by the photoelectric conversion element in each pixel issmall. Hence, the incident light amount tends to be smaller.

To solve this problem, an image sensing apparatus in which a pluralityof adjacent pixels (photoelectric conversion elements) share anelectrical function has been proposed (Japanese Patent Laid-Open No.10-256521). In this case, the element layout relationship near thephotoelectric conversion element may change depending on the adjacentpixel.

An image sensing apparatus sometimes has defective pixels. A defectivepixel outputs a signal (abnormal signal) that is largely different froma normal signal. Following methods of correcting such an abnormal signalhave been proposed.

Japanese Patent Laid-Open No. 2001-045382 has proposed a defective pixelcorrection method for an image sensing apparatus in which a plurality ofphotoelectric conversion elements share an amplifying element. As shownin FIG. 17, four photodiodes all, a12, a21, and a22 each functioning asa photoelectric conversion element share an amplifying element MSF. Ifone of the photodiodes is defective, the signal from the defective pixelis interpolated and then added.

An image sensing apparatus sometimes generates vertical stripe-shapednoise depending on its read circuit. A technique of solving this problemis known, which detects a fixed pattern noise component that reflectsvariations in the row direction of a read transistor by using anon-effective pixel having no photodiode as a dummy line, therebycanceling the fixed pattern noise component (Japanese Patent Laid-OpenNo. 2005-176061).

In the arrangement of Japanese Patent Laid-Open No. 10-256521, if adefective pixel signal is corrected by simply using the signals ofneighboring pixels, as in general practice, the dark current differencemay affect the result.

This is because the dark current affects the signals in differentmanners depending on the difference in the pixel structure. It isimpossible to appropriately correct a signal when the influence of thedark current is large.

In the method disclosed in Japanese Patent Laid-Open No. 2001-045382 aswell, a signal is interpolated using neighboring pixels and then added.In signal interpolation, however, the variations in the dark currentcaused by the difference in the pixel structure are not taken intoconsideration.

Particularly in signal correction in an optical black region (OB region)for outputting a black reference signal, the influence of the variationsin the dark current is larger than in signal correction in an effectivearea for forming an image signal.

Japanese Patent Laid-Open No. 2005-176061 discloses a technique ofcanceling vertical stripe-shaped fixed pattern noise. However, thistechnique does not consider fixed pattern noise generated by pixelstructures in an image sensing apparatus including a plurality of pixelshaving different pixel structures.

SUMMARY OF THE INVENTION

The present invention provides for satisfactorily correcting a signaleven when the dark current varies between pixels.

The present invention also provides for accurately canceling fixedpattern noise generated by an unbalanced electrical characteristic of aread circuit or a circuit included in a pixel in an image sensingapparatus including a plurality of pixels having different pixelstructures.

According to the first aspect of the present invention, there isprovided an image capturing system comprising: a signal correction unitwhich corrects a signal output from a defective pixel in an opticalblack region having a plurality of pixels for outputting a darkreference signal, on the basis of a signal output from a normal pixel;wherein the optical black region has a plurality of pixel blocks, eachof the plurality of pixel blocks has a plurality of pixels eachincluding at least one element which has the same function as in theremaining pixels and which has a relative position different from theremaining pixels, and the signal correction unit corrects the signaloutput from the defective pixel in the optical black region on the basisof a signal output from a normal pixel which is included in anotherpixel block different from the pixel block of the defective pixel in theoptical black region and includes at least one element having the samefunction and same relative position as in the defective pixel.

According to the second aspect of the present invention, there isprovided an image capturing system comprising: a signal correction unitwhich corrects a signal output from a defective pixel in an opticalblack region having a plurality of pixels for outputting a darkreference signal, on the basis of a signal output from a normal pixel;wherein the optical black region has a plurality of pixel blocks, eachof the plurality of pixel blocks has a plurality of pixels eachincluding a plurality of transistors which have the same functions as inthe remaining pixels and which have relative positions different fromthe remaining pixels, and the signal correction unit corrects the signaloutput from the defective pixel in the optical black region on the basisof a signal output from a normal pixel which is included in anotherpixel block different from the pixel block of the defective pixel in theoptical black region and includes a plurality of transistors having thesame functions and same relative positions as in the defective pixel.

According to the third aspect of the present invention, there isprovided an image capturing system comprising: a signal correction unitwhich corrects a signal output from a defective pixel in an opticalblack region having a plurality of pixels for outputting a darkreference signal, on the basis of a signal output from a normal pixel;wherein the optical black region has a plurality of pixel blocks, eachof the plurality of pixel blocks includes a plurality of pixels havingdifferent pixel structures, and the signal correction unit corrects thesignal output from the defective pixel in the optical black region onthe basis of a signal output from a normal pixel which is included inanother pixel block different from the pixel block of the defectivepixel in the optical black region and has the same pixel structure as inthe defective pixel.

According to the fourth aspect of the present invention, there isprovided a signal processing circuit having a signal correction unitwhich corrects a signal output from a defective pixel in an opticalblack region having a plurality of pixels for outputting a darkreference signal, on the basis of a signal output from a normal pixel,wherein the optical black region has a plurality of pixel blocks, eachof the plurality of pixel blocks has a plurality of pixels eachincluding at least one element which has the same function as in theremaining pixels and which has a relative position different from theremaining pixels, and the signal correction unit corrects the signaloutput from the defective pixel in the optical black region on the basisof a signal output from a normal pixel which is included in anotherpixel block different from the pixel block of the defective pixel in theoptical black region and includes at least one element having the samefunction and same relative position as in the defective pixel.

According to the fifth aspect of the present invention, there isprovided a signal processing circuit having a signal correction unitwhich corrects a signal output from a defective pixel in an opticalblack region having a plurality of pixels for outputting a darkreference signal, on the basis of a signal output from a normal pixel,wherein the optical black region has a plurality of pixel blocks, eachof the plurality of pixel blocks includes a plurality of pixels havingdifferent pixel structures, and the signal correction unit corrects thesignal output from the defective pixel in the optical black region onthe basis of a signal output from a normal pixel which is included inanother pixel block different from the pixel block of the defectivepixel in the optical black region and has the same pixel structure as inthe defective pixel.

According to the sixth aspect of the present invention, there isprovided a signal processing method of an image sensing apparatus forcorrecting a signal output from a defective pixel in an optical blackregion having a plurality of pixel blocks each including a plurality ofpixels for outputting a dark reference signal, each of the plurality ofpixels including at least one element which has the same function as inthe remaining pixels and which has a relative position different fromthe remaining pixels, the method comprising: a first step of determiningwhether a signal output from a target pixel is an abnormal output or annormal output; a second step of, when it is determined in the first stepthat the signal output from the target pixel is a normal output,recording the signal output from the target pixel, and an address of thetarget pixel as an address of a normal pixel; a third step of, when itis determined in the first step that the signal output from the targetpixel is an abnormal output, determining the address of the target pixelas an address of a defective pixel; and a fourth step of correcting,based on the address of the normal pixel obtained in the second step andthe address of the defective pixel obtained in the third step, thesignal output from the defective pixel in the optical black region onthe basis of a signal output from a normal pixel which is included inanother pixel block different from the pixel block of the defectivepixel in the optical black region and includes at least one elementhaving the same function and same relative position as in the defectivepixel.

According to the seventh aspect of the present invention, there isprovided a signal processing method of an image sensing apparatus forcorrecting a signal output from a defective pixel in an optical blackregion having a plurality of pixel blocks each including a plurality ofpixels for outputting a dark reference signal, the plurality of pixelshaving different pixel structures, the method comprising: a first stepof determining whether a signal output from a target pixel is anabnormal output or an normal output; a second step of, when it isdetermined in the first step that the signal output from the targetpixel is a normal output, recording the signal output from the targetpixel, and an address of the target pixel as an address of a normalpixel; a third step of, when it is determined in the first step that thesignal output from the target pixel is an abnormal output, determiningthe address of the target pixel as an address of a defective pixel; anda fourth step of correcting, based on the address of the normal pixelobtained in the second step and the address of the defective pixelobtained in the third step, the signal output from the defective pixelin the optical black region on the basis of a signal output from anormal pixel which is included in another pixel block different from thepixel block of the defective pixel in the optical black region and hasthe same pixel structure as in the defective pixel.

According to the eighth aspect of the present invention, there isprovided an image capturing system comprising: a signal correction unitwhich corrects a signal output from an effective pixel including aphotoelectric conversion element in an effective area on the basis of asignal output from a non-effective pixel including no photoelectricconversion element in a non-effective area, wherein the effective areahas a plurality of effective pixel blocks, each of the plurality ofeffective pixel blocks has a plurality of effective pixels eachincluding at least one element except the photoelectric conversionelement, which has the same function as in the remaining effectivepixels and which has a relative position different from the remainingeffective pixels, the non-effective area has a plurality ofnon-effective pixel blocks, each of the plurality of non-effective pixelblocks has a plurality of non-effective pixels each including at leastone element except the photoelectric conversion element, which has thesame function as in the remaining non-effective pixels and which has arelative position different from the remaining non-effective pixels, andthe signal correction unit corrects the signal output from the effectivepixel in the effective area on the basis of a signal output from anon-effective pixel which is included in the non-effective area andincludes at least one element except the photoelectric conversionelement, which has the same function and same relative position as inthe effective pixel.

According to the present invention, it is possible to satisfactorilycorrect a signal even when the dark current varies between pixels.

According to the present invention, it is also possible to accuratelycancel fixed pattern noise generated by an unbalanced electricalcharacteristic of a read circuit or a circuit included in a pixel in animage sensing apparatus including a plurality of pixels having differentpixel structures.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view showing a pixel layout so as to explain anexample of a pixel layout according to the present invention;

FIG. 2 is an equivalent circuit diagram of an image sensing apparatusaccording to Working Example 1;

FIG. 3 is a plan view for explaining the pixel layout of the imagesensing apparatus according to Working Example 1;

FIG. 4 is a sectional view taken along a line X-X′ in FIG. 3;

FIG. 5 is a graph for explaining the dependence of a dark signal levelon the pixel structure;

FIG. 6 is a block diagram of an image capturing system according toWorking Example 1;

FIG. 7 is a flowchart illustrating signal correction according toWorking Example 1;

FIG. 8 is a conceptual view showing a pixel layout so as to explain apixel layout according to Working Examples 2 and 3;

FIG. 9 is a block diagram of an image sensing apparatus according toWorking Examples 2 and 3;

FIG. 10 is a conceptual view showing a pixel layout so as to explain asignal correction method according to Working Example 2;

FIG. 11 is a conceptual view showing a pixel layout so as to explain asignal correction method according to Working Example 3;

FIG. 12 is a conceptual view for explaining a pixel layout according toWorking Example 4;

FIG. 13 is an equivalent circuit diagram showing a pixel arrangementaccording to Working Example 5;

FIG. 14 is a conceptual view showing a pixel layout so as to explain asignal correction method according to Working Example 5;

FIG. 15 is a conceptual view showing a pixel layout so as to explain asignal correction method according to Working Example 6;

FIG. 16 is a block diagram showing an example of an image capturingsystem according to the present invention; and

FIG. 17 is an equivalent circuit diagram for explaining a backgroundart.

DESCRIPTION OF THE EMBODIMENTS

The present invention relates to signal processing of defective pixelsin an image sensing apparatus and, more particularly, to signalcorrection in an image capturing system having an optical black regionincluding a plurality of pixels having different pixel structures.

Terms to be used in the present invention will be defined. An opticalblack region has a plurality of pixel blocks. A pixel block is a unitformed by a plurality of pixels each including one or more elements(e.g., a photoelectric conversion element, a transfer transistorincluding a transfer gate, and the like) which have the same functionsas in the remaining pixels but a different relative positionalrelationship. A pixel is a minimum unit for forming an image andincludes at least a photoelectric conversion element (e.g., aphotodiode).

The plurality of pixels included in each pixel block may share, forexample, an amplifying element (e.g., amplification transistor) orindividually have amplifying elements.

In the former case, each pixel of a pixel block may include a transfertransistor in addition to a photoelectric conversion element. At thistime, each pixel block may be a unit including a plurality of pixelswhich have different relative positional relationships between, forexample, the amplifying element shared by the plurality of pixels andthe transfer transistors included in the pixels. Alternatively, eachpixel block may be a unit including a plurality of pixels which havedifferent relative positional relationships between, for example, thephotoelectric conversion elements and the transfer transistors includedin the pixels.

In the latter case, each pixel of a pixel block may include a transfertransistor in addition to a photoelectric conversion element, or atransfer transistor and an amplifying element in addition to aphotoelectric conversion element. The amplifying element may include anamplification transistor and also a reset transistor and a selectiontransistor. At this time, each pixel block may be a unit including aplurality of pixels which have different relative positionalrelationships between, for example, the photoelectric conversionelements and other elements included in the pixels.

An optical black region has a plurality of pixel units. A pixel unit isa unit formed by a plurality of pixels each including one or moreelements (e.g., a photoelectric conversion element, a transfertransistor, and the like) which have the same functions and samerelative positional relationship as in the remaining pixels.

The image sensing apparatus has a signal correction unit which correctsthe signal from a defective pixel in an optical black region based onthe signal from a normal pixel which is included in a pixel blockdifferent from that of the defective pixel and includes one or moreelements whose relative positional relationship is the same as in thedefective pixel.

Even when a defective pixel exists in an optical black region, the abovearrangement can correct the defective pixel while reducing the influenceof variations in the dark current.

Variations in the dark current between pixels including one or moreelements whose relative positional relationship is different will bedescribed here. When the relative positional relationship between one ormore elements included in a pixel changes, the influence of the darkcurrent on a signal also changes. For example, the vicinity of a elementisolation region or the surface of a semiconductor substrate includesmany crystal defects and readily generates a dark current. The darkcurrent changes depending on the relative positional relationshipbetween these crystal defects and a signal charge channel that affects asignal.

That is, when the relative positional relationship between one or moreelements changes, the relative positional relationship between theelements and the number of crystal defects serving as a dark currentgeneration source changes. This makes the dark current vary.

Hence, if the signal from an abnormal pixel is corrected using thesignal of a pixel which is close to the abnormal pixel but whose darkcurrent differently affects the signal, the signal is affected by thevariations in the dark current and cannot be corrected accurately.

On the other hand, the arrangement of the present invention cansatisfactorily correct a signal while reducing the dark current. Theeffect is particularly large when the arrangement is used to correct asignal largely affected by the dark current. The arrangement caneffectively be applied to correct a defective pixel in an optical blackregion (OB region) because the original signal is small, and theinfluence of the dark current on the signal is large.

The embodiment of the present invention will be described next in detailwith reference to the accompanying drawings. FIG. 1 is a conceptual viewshowing a pixel layout so as to explain the present invention. Theelements included in the pixels of rows A, B, C, and D have differentrelative positional relationships. These pixel rows are repeatedlyarranged. A plurality of pixels included in the rows A, B, C, and D andarranged in the same column form one pixel block.

In this arrangement, for example, when B22 of a row B2 is a defectivepixel, it is corrected using the signal of B12 which is included in arow B1 of a different pixel block and has the same relative positionalrelationship as in B22. This enables to correct a signal while reducingthe influence of the variations in the dark current caused by thedifferent relative positional relationship.

The present invention will be described below based on detailed workingexamples. The present invention is not limited to these workingexamples, and appropriate changes and combinations can be done withinthe above-described scope of the present invention.

WORKING EXAMPLE 1

In this example, four transfer gates share an amplifying element. Thepotential of the gate of an amplification transistor functioning as acommon amplifying element is changed on the basis of charges from thefour transfer gates.

In this example, a plurality of transfer transistors (including transfergates) share a reset transistor and a pixel selection transistor. FIG. 2is an equivalent circuit. FIG. 3 is a view showing the pixel layout.FIG. 4 is a sectional view taken along a line X-X′ in FIG. 3. Thesedrawings illustrate only specific pixels. However, a number of pixelsmay be arranged in the row and column directions.

The circuit arrangement of the upper left pixel block (A11 to D11 shownin FIG. 1) will be described with reference to FIG. 2. The remainingpixel blocks have the same circuit arrangement as that of the upper leftpixel block.

Photodiodes DA11 to DD11 have the same arrangement as that of aphotoelectric conversion element in an effective area. In some cases,the region of the same conductivity type as signal charges isunnecessary (an n-type region is not always arranged for electrons).Each of transfer transistors MTXA11 to MTXD11 transfers charges to theinput portion of the amplification transistor. A semiconductor regionwhich has the same conductivity type as the signal charges and isarranged in the semiconductor substrate is usable as the input portionof the amplification transistor. A reset transistor MRES11 sets at leastthe potential of the input portion of the amplification transistor at areference potential. An amplification transistor MSF11 amplifies thesignal charges. A selection transistor MSEL11 selectively reads out theamplified signal.

Each of control lines PTX_A1 to PTX_D1 applies a pulse to the controlelectrode of a corresponding one of the transfer gates. Control linesPRES1 and PSEL1 apply pulses to the control electrodes of the resettransistor and selection transistor, respectively.

The four pixels including the transfer transistors MTXA11 to MTXD11(including transfer gate; i.e. gate electrode) form one pixel block. Thepixels share MSF11, MRES11, and MSEL11. A plurality of such pixel blocksare arranged in the row and column directions. A common control linecontrols transfer transistors MTXA11 corresponding to DA in the pixelblocks repeatedly arranged in the column direction. The common controlline also controls the transfer transistors MTXB11 to MTXD11corresponding to DB, DC, and DD.

The layout arrangement of the upper left pixel block (A11 to D11 shownin FIG. 1) will be described with reference to FIG. 3. The remainingpixel blocks have the same layout arrangement as that of the upper leftpixel block.

Referring to FIG. 3, wide diagonal-hatched regions indicate diffusionregions corresponding to the photoelectric conversion elements DA11 toDD11. Cross-hatched regions indicate semiconductor regions serving asthe electrode regions of the transistors MTXA11 to MTXD11, MSF11,MRES11, and MSEL11. Narrow diagonal-hatched regions indicate the gateelectrodes of the transistors MTXA11 to MTXD11, MSF11, MRES11, andMSEL11. Solid regions indicate the contact regions to theinterconnections of the upper layer. The remaining regions are basicallythe element isolation regions.

Each region defined by the horizontal and vertical lines corresponds toa pixel. The plurality of elements included in the pixels of the pixelrows A, B, C, and D have different relative positional relationships.

In this example, the plurality of elements included in a pixel are thetransfer transistor MTX, amplification transistor MSF, reset transistorMRES, and selection transistor MSEL. The transfer transistors have thesame positional relationship in the plurality of pixels to individuallytransfer charges to the input portion of the amplification transistor.

FIG. 4 is a conceptual sectional view of the element. Reference numeral401 denotes a p-type semiconductor region. An n-type semiconductorregion 402 accumulates signal charges. A p-type semiconductor region 403reduces a dark current. The semiconductor regions 401, 402, and 403 forma diode corresponding to a photoelectric conversion element in aneffective area. The n-type semiconductor region 402 may have a volumesmaller than the corresponding region in the effective area or may beomitted.

A transfer gate 404 (serving as the gate electrode of a transfertransistor) transfers charges. An n-type semiconductor region (floatingdiffusion region) 405 receives charges and functions as the inputportion of the amplification transistor. A element isolation region 406isolates adjacent elements. The regions 402, 404, and 405 form, forexample, the transfer transistor MTXA11.

It should be noted that the conductivity types in FIG. 4 may be reversedto handle holes as signal charges.

The relative positional relationship between the plurality oftransistors included in each pixel and the relative positionalrelationship between the plurality of transistors and the elementisolation region and the like are largely different between DA, DB, DC,and DD.

More specifically, a pixel block which includes a plurality of transfergates sharing a specific element has four pixels having differentrelative positional relationships between the plurality of elementsincluded in the pixels. A region that accumulates charges and a channelserving as a charge moving path are easily affected by the dark current.

FIG. 5 shows an example of the dependence of a dark signal on the pixelstructure. The optical black region basically has no charges byphotoelectric conversion. Hence, the variations in the dark signal shownin FIG. 5 are largely affected by the variations in the dark currentcaused by the different relative positional relationships between theplurality of elements included in the pixels. As is apparent from FIG.5, the outputs from the pixels A, B, C, and D are largely different.

If a defective pixel is corrected using the signal of an adjacent pixelwhich has relative positional relationship different from that of thedefective pixel without considering the relative positionalrelationships between the plurality of elements included in the pixels,no satisfactory correction result can be obtained because the pixel usedfor correction outputs a different dark signal. To the contrary, when adefective pixel is corrected using the signal of a pixel which is notclose to the defective pixel but includes a plurality of elements havingthe same functions and the same relative positional relationship as inthe defective pixel, the influence of the variations in the dark currentcan be reduced.

In this example, a defective pixel is corrected based on the signal froma normal pixel which is included in a pixel block different from that ofthe defective pixel and includes a plurality of elements whose relativepositional relationship is the same as in the defective pixel. Morespecifically, when the pixel A21 outputs an abnormal signal, it can becorrected using the signal of the pixel A11 while reducing the influenceof the variations in the dark current.

FIG. 6 is a block diagram of an image capturing system according to thisexample. A sequence of detecting an abnormal signal from, for example, adefective pixel and correcting it using the signal of a different pixelwill be described. The image capturing system includes the followingconstituent elements.

An image sensing apparatus 2 functions as an image signal forming unitwhich outputs an image signal. An A/D conversion unit 3 converts theanalog signal output from the image sensing apparatus 2 into a digitalsignal. Reference numeral 4 denotes a signal correction unit. A firstsignal processing unit 5 executes signal processing after defectcorrection.

A detailed sequence will be described with reference to FIG. 7.

First, the signal correction unit 4 determines whether the signal from atarget pixel is an abnormal output or normal output (first step S1). Ifthe signal is a normal output, the signal correction unit 4 records thesignal of the normal output pixel and the address information (pixelgroup information) of the normal output pixel (second step S2). Thesignal correction unit 4 directly outputs the signal of the normaloutput pixel to the first signal processing unit 5 (step S3).

On the other hand, if the signal is an abnormal output, the signalcorrection unit 4 determines the address information (pixel groupinformation) of the abnormal output pixel (third step S4). In accordancewith the determination result in the third step S4, the signalcorrection unit 4 selects a corresponding pixel group by referring tothe memory based on the address information (pixel group information)obtained in the second step S2 (step S5). The signal correction unit 4corrects the signal of the abnormal output pixel using the signal of thecorresponding pixel group (fourth step S6). The signal correction unit 4records the signal of the abnormal output pixel and the addressinformation (pixel group information) of the abnormal output pixel (stepS7). The signal correction unit 4 outputs the signal of the abnormaloutput pixel to the first signal processing unit 5 (step S8). As theaddress information (pixel group information), at least informationabout the pixel structure is recorded.

The signal correction unit 4 shown in FIG. 6 will be described in moredetail. The signal correction unit 4 has a defective pixel detectionunit 4 a which detects whether an input signal is an abnormal signaloutput from, for example, a defective pixel. For example, a differentialcircuit compares a reference value with a signal output from a pixel,and the defective pixel detection unit 4 a detects a defective pixelbased on the comparison result. When the defective pixel detection unit4 a determines that the signal is abnormal and thereby a pixel defectexists, the defective pixel detection unit 4 a sends the signal to thesecond signal processing unit 4 b, and the second signal processing unit4 b of the succeeding stage corrects the signal. When the defectivepixel detection unit 4 a determines that the signal is normal, thedefective pixel detection unit 4 a sends the signal not to the secondsignal processing unit 4 b but to the first signal processing unit 5.Simultaneously, the signal and the address information of the pixel arestored in a memory 4 c. The signal stored in the memory 4 c is used tocorrect an abnormal signal output from another pixel later.

A signal is recorded in the memory together with its address informationto make it possible to discriminate the pixel structure of the pixelwhich has output the signal.

A CPU (not shown) executes the series of control processes for thememory and signal processing units such that an abnormal signal iscorrected using the signal from a pixel which has the same pixelstructure as the pixel that has output the abnormal signal.

For signal correction, a signal may be interpolated by directlysubstituting the pixel signal. Alternatively, a pixel signal to besubstituted may be corrected and then substituted.

As described above, according to the arrangement of this example, it ispossible to correct a signal while reducing the influence of thevariations in the dark current in an image capturing system having aplurality of pixels with different pixel structures. That is, it ispossible to satisfactorily correct a signal while reducing the influenceof the variations in the dark current in an image capturing systemhaving an optical black region including a plurality of pixels withdifferent pixel structures.

WORKING EXAMPLE 2

In this example, a signal correction method related to an optical blackpixel for outputting a black reference signal (dark reference signal)will be described. FIG. 8 shows the pixel layout of an image sensingapparatus.

Referring to FIG. 8, reference numeral 1 denotes a pixel; 10, a first OBregion (vertical OB region); 11, a second OB region (horizontal OBregion); and 12, an effective area adjacent to the optical black regions10 and 11. A dark current is corrected by generating a black referencesignal using these OB regions. When a dark signal distribution (verticalshading) generated by the influence of a dark current exists in thevertical direction, the shading can be reduced by executing correctionusing signals from the horizontal OB region 11. When a dark signaldistribution (horizontal shading) generated by the influence of a darkcurrent exists in the horizontal direction, correction can be done usingsignals from the vertical OB region 10.

FIG. 9 is a block diagram of the image sensing apparatus. WorkingExample 2 is different from Working Example 1 in that the signals from aplurality of pixels (a pixel group included in at least part of a pixelunit) are averaged before they are recorded in a memory. The signalsfrom normal pixels, or the signals from normal pixels and signalsobtained by correcting abnormal signals are averaged. The pixel signalsare recorded in the memory together with address informationrepresenting the pixel line to which the plurality of averaged pixelsignals belong.

In this example, a case in which a defective pixel FP1 exists in thehorizontal OB region 11 will be described. FIG. 10 is an enlarged viewof the horizontal OB region 11 in FIG. 8. The same arrangement as inWorking Example 1 is usable as a detailed pixel layout. Referring toFIG. 10, when the defective pixel FP1 is detected in a pixel unitAR_(B2) of a row B2, the signal of the defective pixel is correctedusing the average value of the signals of a pixel group (e.g., allpixels) included in a pixel unit AR_(B1) of a row B1. The memory recordsthe average signal from the pixel group and information representing thepixel row including that pixel group. Hence, a second signal processingunit 4 b corrects the signal based on the address information in thememory in accordance with an instruction from a CPU (not shown).

In this example, the memory records the average value of the signalsfrom a pixel group included in at least part of a pixel unit. Thisfurther facilitates the memory configuration as compared to WorkingExample 1.

WORKING EXAMPLE 3

This example is related to a signal correction method when a pixeldefect is generated in a vertical OB region 10. This example can alsouse the block diagram of the image sensing apparatus in FIG. 9.

FIG. 11 is an enlarged view of the vertical OB region 10. The samearrangement as in Working Example 1 is usable as a pixel layout, as inWorking Example 2.

In Working Example 3, a pixel row is further divided into a plurality ofregions (block 1, block 2, . . . ) in the horizontal direction. Thesignals from a plurality of pixels in each region are averaged andrecorded in a memory. Simultaneously, pieces of address informationrepresenting the pixel row and the divided regions are recorded.

An averaging processing unit 4 d in FIG. 9 adds the signals from aplurality of pixels in each region divided in the horizontal direction,calculates the average value, and records it in a memory 4 c togetherwith address information. The memory 4 c records the average signal ofeach region divided in the horizontal direction for each pixel row. Apixel signal determined by a defective pixel detection unit 4 a as adefective pixel is corrected by a second signal processing unit 4 b. Thesecond signal processing unit 4 b corrects the signal by acquiring theaverage value of a region including a pixel having the same pixelstructure but included in a different pixel block.

For example, the region of a pixel group included in part of a pixelunit AR_(B2) of a row B2 is represented by AR_(B22) in FIG. 11. Theregion of a pixel group included in part of a pixel unit AR_(B1) of arow B1 is represented by AR_(B12). When a defective pixel FP2 exists inthe pixel group region AR_(B22), the signal is corrected using theaverage signal of the pixel group region AR_(B12) which is included inpart of another pixel unit and also includes a pixel having one or moreelements which have the same functions and the same relative positionalrelationship as in the defective pixel FP2. After signal correction ofthe defective pixel FP2 of the row B2, the average value of the pixelsignals of the pixel group region AR_(B22) of the row B2 is alsorecorded in the memory.

According to this example, it is possible to obtain the accurate blacklevel of each column of the vertical OB region 10 and executesatisfactory correction.

WORKING EXAMPLE 4

In this example, a pixel block formed by a plurality of pixels includingphotodiodes which serve as photoelectric conversion elements and havedifferent areas will be described as an example of a pixel blockincluding different pixel structures. As the block diagram of the imagecapturing system, the same arrangement as in FIG. 6 is usable. Theprocess sequence shown in FIG. 7 is usable.

FIG. 12 is a plan view of an image sensing apparatus of this example.Photodiodes PD11 to PD33 function as photoelectric conversion elements.Other elements such as transistors and electrodes included in the pixelsare not illustrated. For example, the pairs of PD11 and PD21, PD12 andPD22, and PD13 and PD23 form pixel blocks. A pixel 1 forms its signalusing the signals from two photodiodes, that is, photoelectricconversion elements having different areas. This arrangement can widenthe dynamic range. When the occupation ratio of the photoelectricconversion element area in a pixel region changes, the influence of thedark current on the signal also changes. If, for example, PD23 is adefective pixel, its signal is corrected using the signal of the pixelPD21 which is a normal pixel in a different pixel block and has the samepixel structure as the defective pixel. This allows correcting a signalwhile reducing the influence of the variations in the dark current evenwhen a pixel block is formed by a plurality of pixels having differentphotoelectric conversion element areas.

The present invention has been described above in detail. In allexamples, the A/D conversion unit or first signal processing unit caninclude the defect correction unit. In addition, the whole arrangementincluding the image capturing system can be formed in one chip.

In the above-described signal correction methods, a defective pixel isdetected after A/D conversion. However, the present invention is notlimited to this. A defective pixel may be detected from an analogsignal. It is also possible to correct an analog pixel signal.

A signal output from an OB region may be clamped as an analog signal.Alternatively, A/D conversion may be executed using the signal outputfrom the OB region as a reference signal of the A/D conversion unit.

The above examples have been explained in association with an amplifyingimage sensing apparatus. However, the present invention is not limitedto this and is also applicable to, for example, a CCD having a pluralityof pixel groups in which the photoelectric conversion elements havedifferent areas.

WORKING EXAMPLE 5

This example is related to a method of accurately canceling fixedpattern noise generated by an unbalanced electrical characteristic of aread circuit or a circuit included in a pixel in an image sensingapparatus including a plurality of pixels having different pixelstructures.

FIG. 13 is an equivalent circuit diagram showing a pixel arrangementaccording to Working Example 5. The pixel array includes an effectivearea EA where a plurality of effective pixels are arranged, and anon-effective area NEA where a plurality of non-effective pixels arearranged. An effective pixel has the same structure as in FIG. 1 andincludes a photoelectric conversion element. On the other hand, anon-effective pixel includes no photoelectric conversion element.

In a non-effective pixel AN11, for example, as shown in FIG. 13, atransfer transistor MTXAN11 connected to an amplification transistorMSFN11 is connected not to a photoelectric conversion element but to aparasitic capacitance CJAN11 of the transistor. In a non-effective pixelBN11, for example, as shown in FIG. 13, a transfer transistor MTXBN11connected to the amplification transistor MSFN11 is connected not to aphotoelectric conversion element but to a parasitic capacitance CJBN11of the transistor. In a non-effective pixel CN11, for example, as shownin FIG. 13, a transfer transistor MTXCN11 connected to the amplificationtransistor MSFN11 is connected not to a photoelectric conversion elementbut to a parasitic capacitance CJCN11 of the transistor. In anon-effective pixel DN11, for example, as shown in FIG. 13, a transfertransistor MTXDN11 connected to the amplification transistor MSFN11 isconnected not to a photoelectric conversion element but to a parasiticcapacitance CJDN11 of the transistor.

That is, four pixels including four transfer transistors correspondingto one amplification transistor form one pixel block. The position of aneffective pixel A11 in effective pixel blocks (A11 to D14) correspondsto that of the non-effective pixel AN11 in the non-effective pixelblocks (AN11 to DN14). These pixels will be compared as an example. Thepositional relationship (relative positional relationship) between theamplification transistor and the transfer transistor in the effectivepixel A11 is the same as that between the amplification transistor andthe transfer transistor in the non-effective pixel AN11.

FIG. 14 is a conceptual view of the pixel layout shown in FIG. 13. Theeffective area EA has a plurality of effective pixel blocks (A11 toD11), (A12 to D12), (A13 to D13), and (A14 to D14). The non-effectivearea NEA has a plurality of non-effective pixel blocks (AN11 to DN11),(AN12 to DN12), (AN13 to DN13), and (AN14 to DN14).

The effective pixel A11 includes a transfer transistor MTXA11. Theeffective pixel B11 includes a transfer transistor MTXB11. The effectivepixel C11 includes a transfer transistor MTXC11. The effective pixel D11includes a transfer transistor MTXD11.

The non-effective pixel AN11 includes the transfer transistor MTXAN11.The non-effective pixel BN11 includes the transfer transistor MTXBN11.The non-effective pixel CN11 includes the transfer transistor MTXCN11.The non-effective pixel DN11 includes the transfer transistor MTXDN11.

In this example, the non-effective pixels AN11 to DN11 are used as adummy line. Pixel signals read out from the dummy line are used as themeasurement signals of the fixed pattern noise component. Not only thefixed pattern noise component but also a random noise component issuperimposed on the pixel signals read out from the dummy line. Tosuppress the random noise component, the signals are read out multipletimes from the dummy line and averaged in each column. In averagingsignals in each column, pixel signals read out from the non-effectivepixels AN11 to AN14 multiple times are averaged in each column, therebygenerating correction data. Similarly, pixel signals read out from thenon-effective pixels BN11 to BN14, CN11 to CN14, and DN11 to DN14multiple times are averaged in each column, thereby generatingcorrection data.

The correction data obtained by averaging the pixel signals read outfrom the non-effective pixels AN11 to AN14 is subtracted from the pixelsignals read out from the effective pixels A11 to A14, thereby cancelingthe fixed pattern noise component. In a similar manner, the correctiondata obtained by averaging the pixel signals read out from thenon-effective pixels BN11 to BN14 is subtracted from the pixel signalsread out from the effective pixels B11 to B14, thereby canceling thefixed pattern noise component. The correction data obtained by averagingthe pixel signals read out from the non-effective pixels CN11 to CN14 issubtracted from the pixel signals read out from the effective pixels C11to C14, thereby canceling the fixed pattern noise component. Thecorrection data obtained by averaging the pixel signals read out fromthe non-effective pixels DN11 to DN14 is subtracted from the pixelsignals read out from the effective pixels D11 to D14, thereby cancelingthe fixed pattern noise component.

As described above, according to this example, fixed pattern noisecaused by, for example, the parasitic capacitance and parasiticresistance depending on the layout relationship between the transistorsof the effective pixels is canceled using the correction data of thedummy line equivalent to the effective pixels. This also enablessatisfactorily canceling the fixed pattern noise unique to the pixelunit.

WORKING EXAMPLE 6

This example is related to a method of accurately canceling fixedpattern noise generated when adding and reading out the signals of aplurality of pixels arranged vertically in each column in an imagesensing apparatus including a plurality of pixels having different pixelstructures.

FIG. 15 is a conceptual view of the pixel layout according to thisexample. The pixel layout includes effective pixels A11 to D34 andnon-effective pixels AN11 to DN34. Rows A, B, C, and D are arranged bythrees. Rows AN, BN, CN, and DN are also arranged by threes.

In this example, a signal processing unit includes a first addition unit(not shown), a second addition unit (not shown), and a signal correctionunit 4 (FIG. 6).

The first addition unit adds signals output from a plurality ofeffective pixels to generate the sum signal of the effective pixels. Forexample, the first addition unit adds the signal output from the firsteffective pixel and the signal output from the second effective pixel inthe effective area. The first addition unit thus generates the first sumsignal of the effective pixels.

The second addition unit adds signals output from a plurality ofnon-effective pixels to generate the sum signal of the non-effectivepixels. For example, the second addition unit adds the signal outputfrom the first non-effective pixel which includes one or more elementshaving the same functions and the same relative positional relationshipas in the first effective pixel, and the signal output from the secondnon-effective pixel equal to the second effective pixel. The secondaddition unit thus generates the first sum signal of the non-effectivepixels.

The signal correction unit 4 corrects the first sum signal of theeffective pixels based on the first sum signal of the non-effectivepixels. For example, the signal correction unit 4 subtracts a signalobtained by averaging the first sum signals of the non-effective pixelsin each column from the first sum signal of the effective pixels,thereby correcting the first sum signal of the effective pixels.

A case in which the signals of three pixels vertically arranged, thatis, three rows are added while alternately skipping the rows will bedescribed here. The sum signal of the first row corresponds to the sumsignal of effective pixels obtained by adding the signals output fromthe row (row A1) of the effective pixels A11 to A14, the row (row C1) ofthe effective pixels C11 to C14, and the row (row A2) of the effectivepixels A21 to A24. The sum signal of the next row corresponds to the sumsignal of effective pixels obtained by adding the signals output fromthe row (row C2) of the effective pixels C21 to C24, the row (row A3) ofthe effective pixels A31 to A34, and the row (row C3) of the effectivepixels C31 to C34. Focus will be placed on the first column. The sumsignal of the first row is obtained by adding the signals output fromthe effective pixels A11, C11, and A21. The sum signal of the next rowis obtained by adding the signals output from the effective pixels C21,A31, and C31. The combinations of the structures of the added pixels aredifferent. Subsequently, for the non-effective pixels, the signalsoutput from the pixels of three rows are added in the same way. First,the sum signal of the non-effective pixels of one row is obtained byadding the signals output from the row (row AN1) of the non-effectivepixels AN11 to AN14, the row (row CN1) of the non-effective pixels CN11to CN14, and the row (row AN2) of the non-effective pixels AN21 to AN24.Next, the sum signal of the non-effective pixels of one row is obtainedby adding the signals output from the row (row CN2) of the non-effectivepixels CN21 to CN24, the row (row AN3) of the non-effective pixels AN31to AN34, and the row (row CN3) of the non-effective pixels CN31 to CN34.The sum signals of the non-effective pixels are used as correction data.More specifically, the signals output from the plurality ofnon-effective pixels are read out multiple times. Sum signals obtainedby combining, of the pixel signals read out multiple times, the signalsof the rows AN, CN, and AN are averaged in each column, therebygenerating correction data. Similarly, sum signals obtained by combiningthe signals of the rows CN, AN, and CN are averaged in each column,thereby generating correction data.

The correction data generated by averaging the sum signals of thenon-effective pixels obtained by adding the signals output from the rowsAN, CN, and AN is subtracted from the sum signal of the effective pixelsobtained by adding the signals output from the rows A1, C1, and A2. Thiscancels the fixed pattern noise component in the sum signal of theeffective pixels. Similarly, the correction data generated by averagingthe sum signals of the non-effective pixels obtained by adding thesignals output from the rows CN, AN, and CN is subtracted from the sumsignal of the effective pixels obtained by adding the signals outputfrom the rows C2, A3, and C3. This cancels the fixed pattern noisecomponent in the sum signal of the effective pixels.

As described above, according to this example, the sum signal ofeffective pixels is corrected based on the sum signal of non-effectivepixels obtained by adding signals output from a plurality ofnon-effective pixels each of which includes one or more elements havingthe same functions and the same relative positional relationship as in acorresponding effective pixel. This enables to satisfactorily cancelfixed pattern noise unique to the combination of the added pixelsignals.

(Application to Digital Camera)

FIG. 16 shows circuit blocks of an image capturing system of the presentinvention which is applied to a camera. A shutter 1001 is located infront of a photographing lens 1002 to control exposure. A stop 1003controls the amount of light as needed so that an image is formed on animage sensing apparatus 1004. The signal output from the image sensingapparatus 1004 is processed by image signal processing circuit 1005 andconverted from an analog signal to a digital signal by an A/D converter(A/D conversion unit) 1006. The output digital signal further undergoesarithmetic processing by a signal processing unit 1007 (including asignal correction unit and a first signal processing unit). Theprocessed digital signal is stored in a memory 1010 or sent to anexternal device via an external I/F 1013. A timing generation unit 1008controls the image sensing apparatus 1004, signal processing circuit1005, A/D converter 1006, and signal processing unit 1007. A globalcontrol unit/arithmetic unit 1009 controls the entire system. To recordan image in a recording medium 1012, the output digital signal isrecorded via a recording medium control I/F unit 1011 controlled by theglobal control unit/arithmetic unit.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefits of Japanese Patent Application No.2007-121835, filed May 2, 2007, and of Japanese Patent Application No.2008-039094, filed Feb. 20, 2008 which are hereby incorporated byreference herein in their entirety.

What is claimed is:
 1. An image capturing system comprising: an opticalblack region having a plurality of pixel blocks, wherein each of theplurality of pixel blocks has a plurality of pixels in a column foroutputting a dark reference signal, each pixel block including at leastone transistor which performs, for each pixel of the pixel block, thesame function, and each pixel having a relative position, which is aposition within the pixel block to which that pixel belongs, differentfrom that of the remaining pixels of the pixel block to which that pixelbelongs; and a signal correction unit which corrects a signal outputfrom a defective pixel of a pixel block of the plurality of pixel blocksin accordance with only a signal output from a normal pixel of a pixelblock of the plurality of pixel blocks, wherein the pixel blockincluding the normal pixel is different from the pixel block includingthe defective pixel and in a same column as the pixel block includingthe defective pixel, wherein the normal pixel, whose signal is used bythe signal correction unit for the correction, has a relative positionwithin the pixel block to which the normal pixel belongs, that is thesame as a relative position of the defective pixel within the pixelblock to which the defective pixel belongs.
 2. The image capturingsystem according to claim 1, wherein the at least one transistorincludes an amplification transistor.